memblock/arm: Fix memblock_region_is_memory() typo

Fix typo in commit dbe3039 ("memblock/arm: Use memblock_region_is_memory()
for omap fb") - it should be memblock_is_region_memory().

Reported-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: ext Grazvydas Ignotas <notasas@gmail.com>
LKML-Reference: <4CABFADA.9020305@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>